1. Field of the Invention
The present invention relates to an energy trapping type piezoelectric filter comprising a substrate having a plurality of oscillating sections.
2. Prior Art
Various energy trapping type piezoelectric filters are known to date. For example, Japanese Patent Kokai No. 7-263999 discloses such a piezoelectric filter.
Referring to FIGS. 1, 2A and 2B of the accompanying drawings, a piezoelectric filter as disclosed in the above patent document comprises a unitary piezoelectric substrate A, which is provided on a surface thereof with two pairs of laterally arranged oscillatory electrodes B and C, of which the inner oscillatory electrodes B are connected to a centrally arranged capacitor electrode D, as shown in FIG. 2A, and on the other surface thereof with a pair of laterally arranged common electrodes E, which are connected to a centrally arranged capacitor electrode F, as shown in of FIG. 2B. The central capacitor electrode F is located oppositely relative to the capacitor electrode D with the piezoelectric substrate A disposed therebetween. Thus, a pair of oscillating sections V are formed by the two pairs of oscillatory electrodes B and C and the common electrodes E while a coupling capacitor G is formed by the capacitor electrodes F and D. In other words, the unitary piezoelectric substrate A carries thereon a pair of lateral oscillating sections V and a central coupling capacitor G. FIG. 3 is an equivalent circuit diagram of the piezoelectric filter.
The above described conventional arrangement, using a pair of lateral oscillating sections V and a central coupling capacitor G carried on a unitary piezoelectric substrate A, however, is accompanied by a problem that the piezoelectric substrate A is inevitably forced to have an oblong profile and the piezoelectric filter has to have a large surface area for carrying the above listed components thereon. Additionally, such a piezoelectric filter is subjected to various restrictions in terms of the profile, the thickness and the regions assigned to the oscillating sections to significantly limit the variable capacitance range, although the piezoelectric filter has only a single coupling capacitor G.
Therefore, it is the object of the present invention to provide an energy trapping type piezoelectric filter that is free from the above identified problem of the conventional piezoelectric filters.